1. Field of the Invention
The present invention relates to a wavelength converting element for generating the second harmonic of incident light, and a method of manufacturing the same.
2. Description of Background Information
In general, a wavelength converting element for generating the second harmonic of incident light, or an SHG (Second Harmonic Generation) element, is designed such that an optical waveguide, which outputs the second harmonic excited by an incident laser beam, is formed in a substrate of a non-linear optical crystal.
Most of SHG elements are prepared by the following method. As shown in FIG. 1, for example, a tantalum (Ta) mask 3 having an opening 2 equivalent to an optical waveguide is formed on a -z face of a lithium niobate (LiNbO.sub.3) crystal which becomes a substrate 1. Then, a proton exchange is performed on a partial area 4 of the LiNbO.sub.3 substrate 1 through the opening 2 as shown in FIG. 2. This proton exchange is conducted at 220.degree. C. for eight hours using a phosphoric acid solution, for example. Next, a potassium (K.sup.+) ion exchange is performed in the area 4 of LiNbO.sub.3 which has undergone the proton exchange, forming an optical waveguide 5 of potassium niobate (KNbO.sub.3) on the LiNbO.sub.3 substrate 1 as shown in FIG. 3. The potassium ion exchange is conducted at 400.degree. C. for eight hours using a potassium nitrate (KNO.sub.3) solution, for example.
If the optical waveguide 5 of KNbO.sub.3 is formed in the LiNbO.sub.3 substrate 1 by the above method, however, the diffusion of the K.sup.+ ions is deep and the shape of the cross section of the optical waveguide 5 is not fixed. Naturally, the mode of light which propagates in the optical waveguide 5 thus formed becomes a multi-order mode. The SHG element using this optical waveguide 5 cannot therefore yield a high converting efficiency.